The present invention relates to a power source circuit for base drive circuits for a transistor inverter.
The main circuitry of a transistor inverter system comprises, as shown in FIG. 1, a converter 1 which is formed of diodes D.sub.1 -D.sub.6 and converts a three-phase AC power from terminals R, S, T to a DC power. An inverter 2 whose arms are formed of power transistors Q.sub.1 -Q.sub.6 receives the DC power from the converter 1 and converts it into a three-phase AC power of a variable frequency. The resultant AC power is fed to a load such as a motor. Inserted between the converter 1 and the inverter 2 are a smoothing capacitor C and a light emitting diode LED for indicating that the converter 1 is producing an output.
For control of the bases of the transistors Q.sub.1 -Q.sub.6 of the inverter 2, four base drive circuits BD.sub.1 -BD.sub.4, shown in FIG. 2, whose power source lines are independent of each other are needed: one BD.sub.4 for the three transistors Q.sub.4 -Q.sub.6 with the emitters connected to each other and three BD.sub.1 -BD.sub.3 for the transistors Q.sub.1, Q.sub.2, Q.sub.3, respectively.
A conventional power source circuit 3 comprises a transformer T having five secondary windings. Outputs from four of the secondary windings are respectively rectified by diode bridges DB and smoothed by smoothing capacitors Co to produce DC voltages V.sub.1 -V.sub.4 used as the power source voltages for the respective base drive circuits. The remaining secondary winding is used to provide a power source voltage Vc for a conduction control circuit, not shown.
The above-described power source circuit 3 has the following problems. First, the transformer T is bulky and cannot be mounted on a printed circuit board, and therefore a considerable number of and lengthy leads are necessary to connect the transformer with the printed circuit board PC on which the base drive circuits and the conduction control circuit are formed. Moreover, each smoothing capacitor Co must be of a large capacitance. This results in a higher manufacturing cost, lower reliability and a larger overall size. For instance, for an inverter of 15 KVA having transistors Q.sub.1 -Q.sub.6 with rated current of 15 A, the capacitance of each capacitor Co must be as large as 2000 .mu.F, and four such capacitors are needed for the four base drive circuits BD.sub.1 -BD.sub.4. This also results in higher cost and larger overall size.